We have previously reported that halothane's reactive oxidative metabolite, trifluoroacetyl halide forms trifluoroacetylated (TFA) covalent adducts within and on the outer surface of hepatocytes, when halothane is administered to rats. These adducts have now been further characterized by various immunochemical procedures and their relationship to halothane-induced hepatotoxicity is being investigated. The major TFA adduct within the cell has been identified as a 54 kD form of microsomal cytochrome P-450. A 54 kD TFA protein was also detected in the plasma membrane fraction of the cell. Immunoperoxidase staining of Western blots of plasma membrane proteins with specific antibodies revealed that cytochrome P-450-54 kD and cytochrome P-450 reductase were present in this fraction of the cell. Immunoelectron microscopic examination of intact hepatocytes confirmed that cytochrome P-450 was a component of the plasma membrane. TFA adducts were also detected in microsomes from patients that were administered halothane. Moreover, the sera of two of six patients that had the fulminant form of halothane-induced hepatitis, contained anti-TFA antibodies. These results show unequivocally that cytochrome P-450 and cytochrome P-450 reductase are plasma membrane components. This importantly implies that the toxicity of many drugs may be due to the formation of reactive and toxic metabolites at this site, where they may cause death either by an acute injury or by an immunologically-based mechanism. In the case of halothane, its reactive trifluoroacetyl halide metabolite may be produced, at least in part, in the plasma membrane where it forms TFA adducts. In susceptible individuals, these adducts maybe recoginized by the immune system as foreign antigens, resulting ultimately on second exposure in an immunologically-based hepatotoxicity.